Hydraulic brake booster



Aug. 14, 1945. w TELZER 2,381,989

HYDRAULIC BRAKE BOOSTER Filed July s; 1943 2 Sheets-Sheet 2 INVENIUB.

- claims.

Patented Aug. 14, 1945 UNITED \STATES PATENT OFFICE Claims.

The invention relates to hydraulic brake boosters and more particularly to a booster for increasing the volume 01 hydraulic fluid transmitted from the master cylinder to the wheel cylinders in a hydraulic braking system, where said master cylinder is of small displacement but capable of producing high pressures.

The object of the invention is to provide a power operated booster to increase the volume of fluid transmitted to the wheel cylinders before the hydraulic pressure in the latter becomes high, and to provide means to transmit the high pressure produced by the master cylinder directly to the wheel cylinders after the booster is incapable of further increasing the volume of hydraulic fluid transmitted to the wheel cylinders.

Another object is toprovide nov'el means to control the power of the booster so that the pressure of the hydraulic fluid transmitted by the booster is in a fixed relation to the hydraulic pressure produced by the master cylinder, and to arrange said novel means in such a way as to become very sensitive to small pressure differences between the master cylinder pressure and the wheel cylinder pressure.

It is also the aim of this invention to make the booster operative in case of failure of the power.

A further aim is to provide means to return the fluid from the wheel cylinders when the brakes are being released.

Other aims and advantages will become ap-' parent by inspection or the drawings submitted for the purpose of illustration and not intended to define the scope of the invention, reference bein had for that purpose to the subjoined In the drawings:

Fig. l is a sectional elevation of the novel booster, connected to a hydraulic braking system illustrated digrammatlcally; and

Fig. 2, a sectional elevation of a modified form, connected to a hydraulic braking system diagrammatically. v

Before explaining the present invention in detail, it is to be understood that the invention isnot limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being carried out or practised in various ways, also it is to be understood that the phraseology' or terminology employed herein is for the purpose of description and not limitation.

Describing now the construction of the invention, Fig. 1 illustrates ahydraulic braking system comprising a master cylinder or pressure producing device I operated by manual powerapplied to foot pedal 2, and wheel cylinders or hydraulic pressure receiving means 3 to actuate the brakes or other similarly operated device.

' 'rectly to the wheel cylinders, in this novel system the booster is interposed in said pressure line, whereby fluid pressure transmitting line 4 leads from the master cylinder l to the booster,

.and from the latter line 3 leads to the wheel cylinders.

The hydraulic booster comprises two housings 6 and I secured together with screws 8 with a diaphragm 3 interposed. The latter fits on a .piston unit comprising a diaphragm plate III, a power piston ll sliding in a cylinder chamber l2, and a small piston I3 sliding in cylinder chamber l4 subject to the hydraulic pressure from the master cylinder. The pistons are provided with seals l3 and I8, and IS, the latter being retained by return spring II. The enlarged portions of housings 6 and l clamped together are part of an expansible chamber motor mechanism having working chambers l3 and I9 separated by diaphragm 3 which is also a valve 23 and line 24, or to the source of vacuum 20 via passage 25, valve 26, chamber 21 and line 28, depending on which valve is open.

Valve 23 is urged against valve seat 23 to close ofl chamber I! from 30 by a conical spring 3| retained in housing 1 by a snap ring 32. The hollow valve stem of valve 23 is slidable in housing 7 and its extremity serves as a valve seat for ball 26 engaged by a rocker arm 33 iulcrumed at 34 and having a socket 35 to rest on a pin 38 extending upwardly from'fltting 31 provided with a fluid passage 38. The upper arm of rocker or lever 33 extends into a hydraulic chamber 39 which is sealed against chamber 21 by a seal 40 seated at the bottom of chamber-39. The upper end 01' rocker arm 33 engages a ,groove in control piston 4| sliding in the same bore as. piston l3 and being urged towards the latter by when the power is insufficient to force piston further into chamber I2, I provide a by-pass valve 44 urged against opening 45 by spring 46 g and piston rod 41 of piston 48 sealed against the atmosphere with a seal 49. 50 represents a modulating spring urging piston 43 towards the leitto keep ball 44 in a seated position against position.

' through line 64 with a checkvalve 65 interposed ing on seal 49. To limit the movement of piston and the hydraulic pressure in chamber 39 act- 46 in yielding to the pressure of the fluid in chamber 39 a stop 6| is provided on plug 62, the

latter serving to retain spring 66. Chamber 39 is in communication with the wheel cylinders through line 53, which also communicates with chamber l2 through line 63, whereby these two lines may be considered a part of the booster unit. The fluid in flowing from chamber l2 to line 53 has to pass check valve 64 raised from its seat by a stepped push rod 66 seated on seal 56 of the piston 51 adapted to slide in housing 6 and urged upwardly by a spring 68 retained by a plug 69 having a stop 60. Piston 81 has a shoulder 6| serving as a stop to arrest the upward movement of the piston after ball 64 is unseated. A small vent hole 62 provides atmospheric pressure under piston 51. A similar hole, it will be noted, is provided to relieve the pressure behind piston 48. Spring 58 must be of suflicient strength to unseat ball 54 when the hydraulic pressure in line 63' resists this action and the pressure in chamber I2 is relieved, yet it must yield to the hydraulic pressure acting on piston 51 as soon as the pressure in chamber I2 is increased sufllciently to apply the brakes.

To permit the fluid transmitted to the wheel cylinders to return to the master cylinder and to insure that chamber I2 is filled first during the return of the booster elements to the starting position, a port hole 63 is provided in cylinder housing 6 and placed to be uncovered by seal l6 when piston II is in the "03 or starting Port 63 communicates with line 4 permitting the flow of fluid from port 63 to line 4 but not vice versa.

In the modification in Fig.2 the control piston responsive to the hydraulic pressure of the master cylinder and to the hydraulic pressure of the wheel cylinders is incorporated in the working piston unit, whereby the valves for controlling the power fluid are more conveniently located in the piston of the expansible chamber motor mechanism. The booster unit in this modified construction comprises a housing 65 having a cylinder bore or chamber 66 in which slides a power piston 61 to serve as a member of the expansible chamber motor mechanism. The end of housing 65 has a cover 66 to form chamber 69 of the expansible chamber motor mechanism. The central portion of cover 68 has a chamber in which slides a piston 1| of small displacement. Chamber 10,is in communication with master cylinder I through line 4. Housing 66 also has a central bore or chamber 12 in which slides a piston 01' large displacement 13 in unison with piston 61. The latter two pistons are bored out. centrally to accommodate piston 1| secured to piston 1| and adapted to slide endwise a short distance, shoulder or collar 14 of piston 1| serving as a stop against ring 15 retained in piston 61 by a snap ring 16.

To direct the power fluid to and from the expansible chamber motor'mechanism, a pair of valves are provided comprising ball 11 urged away from seat 18 by spring 19 to provide communication between chambers 66 and 69 throughhole 80, and ball 3| urged into a seated position by spring 82 in chamber 63 to which flexible hose 84 is connected to carry to it atmospheric pressure from source 22. Springs 19 and 32 are opposed by a plate 86 having a central hole to fit freely on piston 1| and to be able to rock on piston 1|. A helical spring 96 urges the piston unit into the "oil" or starting position. 61, 86 and 99 represent seals between piston 1| and 13, between 13 and housing 65, and between'piston 61 and housing 65, respectively. The means for returning the hydraulic fluid from the wheel cylinders to the chamber of larger displacement is the same as in Fig. 1, consisting mainly of check valve 54 and piston 51. The means for preventing the direct flow of fluid from the master cylinder to the wheel cylinders is conveniently located in housing 65 and comprises the ball 44 urged against hole 90 by a modulating spring 9| seated on plug 92 and acting through the intermediary of piston 93 and piston rod 94. Ball 44 is also urged in a seated position by a light spring 95 which serves to secure seal 96 to piston 93. Piston 33 is acted upon by the hydraulic pressure in the wheel cylinders or the hydraulic pressure in chamber 12.

Having thus described the construction I shall now explain the operation of the same. Assuming first that the brakin system is at rest or in the o position as illustrated in Fig. 1, it will be noted that chambers I6 and I9 are evacuated, the pressure in chamber l9 being able to relieve itself through passage 25 past ball 26 into source of low pressure 26, which may be the intake manifold of an internal combustion engine.

Supposing now that the operator depresses brake pedal 2 to produce a hydraulic pressure in master cylinder the pressure is communicated to chamber l4 urging to move piston i3 and II towards the left. The pressure also acts on piston 4| moving it towards the right to rock lever 33 about its fulcrum so that the lower becomes unseated and spring 3| compresses.

Communication is established between chamber also to chamber 39 in order to act on the back of piston 4| in opposition to. the manually produced pressure in chamber l4, there being clearance around the right hand end of the piston 4| as stated. As. soon as the hydraulic pressure in the power circuit whose pressure is generated in chamber |2 approaches the pressure in chamber H or the manual circuit, spring 42 urges piston 4| towards the left, helped by spring 3|, to seat valve 23 or to reduce the opening betweenvalve 23 and seat 29 to such an extent as to maintain a certain pressure difierential between the manual circuit and the power circuit, the pressure in chamber |4 being slightly higher than in chamber 39 in order to overcome spring 42. Due to this slight pressure differential the actual pressure causing friction of seal 43is very low, resulting in a high sensitivity of these pressure responsive means. Observing now the efiect of the pressure in chamber l2 on piston 51, spring 68 yields so that piston 61 comes to rest on stop 60 whereby ball 54 is free to become seated should the flow of hydraulic fluid from chamber l2 to wheel cylinders 3 cease.

left at a rate proportional to the volume of fluid forced into chamber M by master cylinder l';--

the pressures are gradually increased as the resistance to further expansion increases. Since the power of the expansible' chamber motor mechanism is limited with a givendiameter of diaphragm land a limited pressure differential between 28 and 22, a certain maximum pressure is reached in chamber l2 which cannot be increased even though valve 23 is unseat'ed. At this stage by-pass valve 44 comes into play. As the hydraulic pressure produced by master cylinder is further increased it unseats ball 44 in overpowering spring 44, spring Ill having been compressed previously due to the hydraulic pressure in chamber 38. Now the hydraulic fluid flows from master cylinder to wheel cylinders 3 through line 4, past check valve 44, via chamber 38, and lines 83 and 8, check valve 84 being closed as the piston unit is stationary. In order to insure proper functioning of control piston 4| to control the valve after piston 48 is forced against stop 8|, it is necessary to select spring 48 of sufficient strength to insure the pressure difierential necessary to move piston 4|. Thus it is necessary that spring 42 yields before spring 48. I

Supposing now that the operator releases the foot pedal with a resultant relief of pressure in line 4 and chamber l4, valve 44 closes and piston v4| moves towards the left by virtue of spring 42 and the existing pressure in chamber 38, thereby allowing valve 23 to close and ball 38 to unseat so that the pressure in chamber i8 is relieved. As the pressure differential between chambers l8 and I8 is decreased, the pressure in chamber |e2 drops accordingly, allowing spring 88 to push piston -81 upwardly and to unseat check valve 84, permitting fluid to return from wheel cylinders 3 to chamber ll, while pistons |2, l8, and I3 return to the oil or starting position. When the starting position is reached, port 83 is uncovered by seal l8 so that hydraulic fluid may return from wheel cylinders 3 to master cylinder I through line 53', chamber l2, port 85, and line 84.

Assuming now that the booster is operated under a condition where power has'failed, i. e., where no pressure differential exists between28 and 22 and operation of valves 23 and 28 is useless, depression of pedal 2 results in a hydraulic pressure transmitted through line 4 to chamber l4 to move piston l3, III and H towards the left, piston 4| coming to rest against housing 1.

Since piston I3 is of smaller displacement than piston II, the hydraulic pressure in chamber I2 is proportionally lower, but sufficient to operate wheel cylinders 3 to apply the brakes. The arrangement of spring 80 and piston 48 serves to keep valve 44 closed so that the pressure in chamber l4 can be built up substantially without being able to overpower spring 88. So the secber I2 is relieved proportionately so that spring 83 opens valve 84 and provides for the return of hydraulic fluid from the wheel cylinders to chamber l2, and from there to the master cylinder after piston H has arrived at the starting position where port 83 is uncovered.

' Describing now the operation of the construction illustrated in Fig. 2, and assuming that the booster unit is in the o position, as shown, both chambers 88 and 88 ofthe expansible chamber motor mechanism are open to vacuum. Valve II is closed by virtue of spring 82, and valve 11 is held open by spring 18, the ball being pressed against plate 88 urging piston 1| towards the right where shoulder 14 rests against ring 1-8, the movement of piston 81 being arrested by cover 88. Spring 82 is of sufllcient stiffness to dominate spring 18 so that the latter is incapable of unseating valve 8|.

Supposing now that the operator depresses pedal 2 producing a hydraulic pressure in master cylinder I, the pressure acts in chamber 18 urg- I ing piston 1| and 1| towards the left, causing a movement of the latter relative to pistons 81 and 13 and thereby pushing ball 11 into a seated position, the upper extremity of plate 88 remaining stationary and the lower end moving towards the left at an increased rate of travel until ball 11 is seated, resulting in a rocking action of plate 88. As, piston 1| moves further towards the left, due to the resistance oflered by ball 11 after it is seated the upper end of plate. 88 moves towards the left to unseatball 8| so that air is admitted from the source of pressure 22 through bore 84 into chamber 88,

. moving piston 81 and 13 towards the left to ondary pressure in chamber 39 is built up, a

pre-determined stage is reached where the hydraulic pressure in chamber 38 acting on piston 48 compresses spring 88 and permits the flow of hydraulic fluid from line 4 past check valve 44 into line 53 vto wheel cylinders 8, check valve 54 becoming seated and piston I being arrested. The pressure loss at the second stage is only slight due to light spring 48, which illustrates the merit of piston 48 and spring 88.

As the operator releases pedal 2 and the pressure in chamber |4 drops, the pressure in chamtravel in unison with pistons 1| and 1|' 11 and 81 to follow piston1l. In the particular embodiment where pistons 1| and 1| are of equal diameter the pressure in chamber 12 is built up to be equal to or slightly less than the pressure in chamber 10. Balance of pressures at the ends of piston 1|-1|' causes valves 11 and 8| to be closed, the pressure in chamber 18 being slightly greater than in chamber 12 to compensate for spring 18. Piston 1| responsive to the hydraulic pressure of the master cylinder urges to increasethe power of the expansible chamber motor mechanism, while piston 1| responsive to the pressure produced by the power operated piston 13 opposes 1| and thereby tends to decrease the power of the expansible chamber motor mechanism.

While pedal 2 is further depressed, chamber 18 increased, and pistons 1| and 1|, 13, and 81 travel towards the left in unison, fluid in chamber 12 is displaced and forced to wheel cylinders 3 passing past check valve84. As the pressure has reached a certain level in chamber 12, spring 58 is compressed and piston, 51 rests on stop 80. A further rise in pressure in chamber." and wheel cylinders 3 causes spring 8| to yield so that light spring 88 becomes the sole means for urging ball 44 in a seated position. Supposing now that full power is applied topiston 81 and that pressure in line 84 unseats ball 44 by overpowering spring 88, allowing fluidto pass from master cylinder I through line 84, pastiball 44, and

through line 5 to wheel cylinders 3, pistons ll, 13 and 61 being stationary and ball 54 seated. When the operator releases pedal 2, pistons II and II move towards the right, allowing valve 8| to seat and H to open so that the entire piston unit may return to the starting position due to spring 86, while valve 54 is held open by spring 58 which expands as soon as the pressure in chamber]! has dropped. In the o position the fluid that had passed from the master cylinder to wheel cylinders 3 through valve 44 ma return again through port 83.

I claim:

1. In a hydraulic system having a hydraulic pressure producing device and hydraulic pressure receiving means, in combination, a booster comprising an expansible chamber of small displacement in communication with said hydraulic pressure producing device, an expansible chamber of larger displacement in communication with said,

hydraulic pressure receiving means, a check valve interposed between said chamber of. larger displacement and said hydraulic pressure receiving means and operative to prevent the flow of hydraulicfluid'irom the latter to the former, mechanical means to transmit the pressure of said expansible chamber of small displacement to said expansible chamber of 1arger displacement so that an increase in volume in said expansible chamber of small displacement enforces a proportionately greater decrease in volume in said expansible chamber of larger displacement, power means to assist to reduce the volume of said expansible chamber of larger displacement proportionately to the hydraulic pressure produced by said hydraulic pressure producing device, valve means to provide a hydraulic passage from said hydraulic pressure producing device to said hydraulic pressure receiving means when a certain pressure is reached, means for opening said check valve when pressure in said chamber of larger displacement is below a predetermined point, and means to provide a passage to return the hydraulic fluid to said hydraulic pressure producing device from said expansible chamber of larger displacement after the pressure in said hydraulic pressure producing device is relieved.

2. The construction as claimed in claim 1, where said valve means consists of a check valve arranged to check the flow of hydraulic fluid from said hydraulic pressure receiving means to said hydraulic pressure producing device, resilient means to urge to keep said check valve closed, and means responsive to the hydraulic pressure communicated to said hydraulic pressure receiving means to urge said resilient means to yield to allow said check valve to open to permit the flow of fluid from said hydraulic pressure producing device to said hydraulic pressure receiving means.

fluid in said cylinder of smaller displacement enforces a proportionately greater decrease in the volume of hydraulic fluid in said cylinder of larger displacement, power means to assist to move said piston in said cylinder of larger displacement with a force in proportion to the hydraulic pressure in said master cylinder, a check valve intermediate said master cylinder and said wheel cylinders arranged to check the flow of fluid from said wheel cylinders to said master cylinder, spring means arranged to urge said check valve in a closed position, means responsive to the hydraulic pressure in said wheel cyl inders to oppose said spring means in order to permit the new of hydraulic fluid from said master cylinder to said wheel cylinders alter a certain pressure is reached, means for opening said first named check valve when pressure in said cylinder of larger displacement is below a predetermined point, and means to provide a passage for the return of fluid from said cylinder of larger displacement to said master cylinder after the hydraulic pressure in said master cylinder is relieved.

. 4. In a hydraulic braking system having a master cylinder operated by the operator and wheel cylindersto operate the brakes, in combination, a booster comprising a cylinder of small displacement in communication with said master cylinder, a cylinder of larger displacement in communication with said wheel cylinders, a check valve interposed between said cylinder of larger displacement and said wheel cylinders to check the flow of fluid from said wheel cylinder to said cylinder of larger displacement, a piston in said cylinder of small displacement, a piston in said cylinder of larger displacement, the former being arranged to actuate the latter so that an increase in volume of hydraulicfluid in said cylinder of smaller displacement enforces a larger displacement in said cylinder of larger displacement to force fluid from the latter to said wheel cylinders, an expansible chamber motor mechanism to assist to move said piston in said cylinder of larger displacement, valve means to direct power fluid to and from said expansible chamber motor mechanism, means responsive to the hydraulic pressure produced by said master cylinder to urge said valve means to increase the power of said expansible chamber motor mechanism, means responsive to the hydraulic pressure produced by said piston in said cylinder of larger displacement to urge said valve means to decrease the 3. In a hydraulic braking system having a mas- 1 ter cylinder operated by the operator and wheel cylinders to operate the brakes, in combination,

. a booster comprising a cylinder of small displacement in communication with said master cylinder, a cylinder of larger displacement in communication with said wheel cylinders, a check valve interposed between said wheel cylinders and said cylinder of larger displacement arranged 'to check the flow of fluid from said wheel cylinders to said cylinder of larger displacement, a piston in said cylinder of small displacement, a piston in said cylinder oi. larger displacement, the former' being arranged to actuate the latter so that an increase in the volume of hydraulic power of vsaid expansible chamber motor mechan1sm,.a normally closedcheck valve in communication with said wheel cylinders on one side and said master cylinder on the other side, and ar ranged to prevent the flow of fluid from said wheel cylinders to said master cylinder, means to yieldingly urge said normally closed check valve to remain in .a closed position, means responsive to the hydraulic pressure of said wheel cylinders to oppose said means that urge said check valve to remain closed, a passage in communication with said master cylinder and said cylinder of larger displacement when the piston in the latter is retracted, and means to provide the return of hydraulic fluid from said wheel cylinders to said cylinder of larger displacement and through said passage to said 'master cylinder after the hydraulic pressure in the latter is relieved.

5. The construction as claimed in claim 4,

urging to unseat said check valve that is interposed between said wheel cylinders and said cylinder of larger displacement, and a piston responsive to the hydraulic pressure in said cylinder of larger displacement opposing said spring to per-,

mit said check valve to remain closed.

6. In a hydraulic braking system having a master cylinder operated by the operator and wheel cylinders to apply the brakes, in combination, a booster comprising a cylinder of -small diameter in communication with said master cylinder,-a cylinder of large diameter in communication with said wheel cylinders through the intermediary of a check valve permitting the flow of hydraulic fluid from said large cylinder to said wheel cylinders but checking the flow in the opposite direction, said cylinders belng opposed and co-axial with respect to each other, a small piston in said small cylinder, a power piston in said large cylinder, said piston in said small cylinder extending through said power piston to be responsive to the hydraulic pressure produced by said master cylinder on one side and to the hydraulic pressure produced by said power piston on the other side, stops to limit the travel of said small piston relative to said power piston, an expansible chamber motor mechanism to actuate said power piston, follow-up valve means responive to the excursions of said small piston relative to said power piston to direct power to said expansible chamber motor mechanism to urge said power piston to follow said small piston to force hydraulic fluid from said cylinder oi. large diameter to said wheel cylinders, means to provide a passage from said master cylinder to said wheel cylinders after a certain I pressure is reached, means for opening said check valve when pressure in said cylinder of larger displacement is below a predetermined point, and means to provide the return of fluid from said cylinder of large diam eter to said master cylinder after the hydraulic pressure in said master cylinder is relieved.

'7. In a hydraulic braking system having a master cylinder operated by the operator and wheel cylinders to operate the brakes, in combination, a booster comprising a cylinder of small displacement, fluid pressure transmitting means between said master cylinder and said cylinder of small displacement, a cylinder of larger displacement co-axial with and opposed to said cylinder of small displacement, fluid pressure transmitting means between said cylinder or larger displacement and said wheel cylinders, a check valve interposed to check the flow of hydraulic fluid from said wheel cylinders to said cylinder of larger diameter but to permit the flow vice versa, pistons in said cylinders connected together to move in unison whereby a certain amount of hydraulic fluid forced into said cylinder of smaller displacement urges a larger displacement of hydraulic fluid in said cylinder of larger displacement to be forced to said wheel cylinders, a double-acting control piston responsive on one side to the hydraulic pressure produced by said master cylinder and on the opposite side to the hydraulic pressure communicated to said wheel cylinders, an expansible' chamber motor mecha nism to assist said pistons, power fluid to operate said expansible chamber motor mechanism, valve means to direct power fluid to and from said expansible chamber motor mechanism, means responsive to the excursions of said control piston to operate said valve means whereby the hydraulic pressure produced by said master cylinder urges to increase the power of said expansible chamber motor mechanism and the hydraulic pressure communicated to said wheel cylinders urges to decrease the power of. said expansible chamber motor mechanism, a check valve intermediate said master cylinder and said wheel cylinders arranged to check the flow of fluid from said wheel cylinders to said master cylinder, yielding means to urge to keep said last mentioned check valve in a closed position, means to establish communication between said wheel cylinders and said cylinder of larger diameter when the pressure in the latter is relieved to permit the return of hydraulic fluid from said wheel cylinders to said cylinder of larger displacement, and means to permitthe flow of hydraulic fluid to said master cylinder from said cylinder of larger displacement when in the starting position but not permitting reverse flow. a

8. The construction as claimed in claim 7, means responsive to the hydraulic pressure of the fluid communicated to said wheel cylinders to oppose said yielding means to provide the flow of fluid from said master cylinder to said wheel cylinders after a certain pressure is reached,- a light auxiliary spring to urge said check valve intermediate said master cylinder and said wheel cylinders in a closedposition to provide a given pressure diii'eren'tial, and spring means to urge said valve means to reduce the power of said expansible chamber motor mechanism.

9. In' a hydraulic braking system having a master cylinder operated by the operator and wheel cylinders to operate the brakes, a booster comprising a relative large cylinder and a relatively small cylinder communicating respectively .piston through the displacement of fluid from said master cylinder into said smaller cylinder, a motor mechanism for moving said larger piston, follow-up control means for causing said motor mechanism to operate said larger piston in proportion to fluid displaced from said master cylinder into said smaller cylinder, a second check valve controlling communication between said master cylinder and said wheel cylinders to prevent the flow of fluid from said wheel cylinders to said master cylinder, yielding means urging said second check valve to closed position and adapted to release said check valve upon the building up of a relatively high master cylinder pressure in proportion to the wheel cylinder pressure, means biasing said first named check valve to open position and yieldable under pressures in said larger cylinder above a predetermined point, and a passage for returning fluid from said larger cylinder to said master cylinder when'said larger piston is in fully retracted position.

10. Apparatus constructed in accordance with claim 9 wherein said last named means comprises a spring biasing said first named check valve to closed position, and a device responsive to pressures in said larger cylinder to oppose said spring whereby said first named check valve is released for movement to closed position when pressures in saiidtlarger cylinder are above a predetermined D0 n 

